This invention relates to a superconducting magnet device and, more particularly, to an improvement in such a device which has a smaller superconducting magnet and a smaller refrigerator and is used, for example, in a monocrystal fostering apparatus, magnetic resonance imaging (MRI) system and the like.
FIG. 1 shows the construction of the conventional superconducting magnet device. In the diagram, a superconducting coil 1 serving as a superconducting magnet is enclosed in an inner chamber 3 in which a cryogen, for example, liquid helium 2 at a very low temperature (e.g., at 4.2.degree. K.) is filled. A cold insulation vessel 4 to hold this superconducting coil 1 into the superconducting state comprises: the inner chamber 3; an outer chamber 5 to enclose this inner chamber 3 ordinarily in the vacuum state; and a plate-like shield member 6 of thermal radiation which is interposed between the inner chamber 3 and the outer chamber 5. Further, the thermal radiation shield member 6 is provided with a tubular shield member 8 to raise the shielding effect thereof. On the other hand, an exciting current is supplied from an external power source 9 for the superconducting magnet through a power lead 10 to the superconducting coil 1. This enables the desired magnetic field to be applied to equipment 16 to which the magnetic field is applied. This equipment is arranged so as to penetrate through the central portion of the cold insulation vessel 4.
On the other hand, under such a situation, the heat penetrates from the outside space at an ordinary temperature (e.g., at 300.degree. K.) into the liquid helium 2 which is held at a very low temperature (e.g., 4.2.degree. K.) due to thermal conduction and thermal radiation through the power lead 10, a low temperature piping 11, a liquid helium transfer pipe 12, the outer chamber 5, the thermal radiation shield member 6, and the inner chamber 3. A part of the liquid helium 2 is ordinarily evaporated due to this penetration heat, so that gaseous helium 13 is generated. This gaseous helium 13 flows inside an outer pipe 14 in which the power lead 10 passes and enters the low temperature piping 11 while cooling (gas-cooling) the power lead 10. A quantity of penetration heat from the power lead 10 is reduced due to this gas-cooling. The gaseous helium 13 enters a helium liquefying apparatus 15 and is converted to liquid helium at a very low temperature (e.g., at 4.2.degree. K.). This liquid helium is put into the inner chamber 3 through the liquid helium transfer pipe 12. As described above, after the helium evaporated by the penetration heat has been cooled by the power lead 10, it is liquefied by the helium liquefying apparatus 15 and is returned to the inner chamber 3. This circulation is repeated, thereby holding the superconducting coil 1 in the superconducting state.
On the other hand, although the conventional superconducting magnet device constructed as described above is suitable for a large superconducting magnet, it is improper for a relatively small superconducting magnet (e.g., the exciting current is about 300 to 500 A and the helium evaporation quantity is about 1 to 2 l/h at a very low temperature) which is used, for example, in a monocrystal fostering (putting-up) apparatus or the like as equipment 16 to which the magnetic field is applied. This is because the helium liquefying apparatus 15 of the conventional type has been developed for use in a large superconducting magnet, and it is not suitable for an apparatus having small refrigerating ability (e.g., the helium evaporation quantity is about 1 to 2 l/h). Therefore, if the ordinary helium liquefying apparatus 15 is employed in the small superconducting magnet, the size and the area occupied by the helium liquefying apparatus 15 will become extremely large as compared with the superconducting magnet. Further, with respect to the manufacturing cost, the cost of the helium liquefying apparatus 15 is much larger than the smaller superconducting magnet, which makes the superconducting magnet device extremely expensive. On the other hand, it is also possible to consider the method whereby the small refrigerator of the conventional type which equivalently corresponds to the capacity of this small superconducting magnet is used, thereby reducing the size and cost of the superconducting magnet device. However, the conventional small refrigerator doesn't have enough refrigerating ability to extinguish the heat penetrated through the cold insulation vessel 4, inner chamber 3, shield members 6 and 8, and outer chamber 5 to further cool the power lead 10. Therefore, ordinarily, a permanent current switch is attached to the superconducting coil and the power lead is made detachable; after the superconducting coil has been excited, the power lead is detached, thereby shutting off the heat which penetrates from the power lead; and the device is operated in the permanent current mode. With such a constitution, the heat is penetrated from the outside due to only the thermal radiation and the thermal conduction from various low-temperature pipes, so that the superconducting magnet can be sufficiently held into the superconducting state even by only the refrigerating ability of the conventional small refrigerator.
However, according to this technique, once the device has entered the permanent current mode, the exciting current is always constant and the current value cannot be varied. For instance, when considering the small superconducting magnet device which is used in the monocrystal pulling-up apparatus, it is required to control the impurity concentration in the monocrystal by changing or controlling the magnetic field strength while the monocrystal is being pulled up. For this purpose, it is necessary to control the magnetic field strength, i.e., the exciting current value. As described above, generally in the equipment using the superconducting magnet device, it is usually demanded that the strength of the magnetic field be applied thereto, i.e., the exciting current value can be varied or controlled.